The Water Sensor attaches to the water line. When you flush the toilet, the Water Sensor detects the vibration and triggers the Bathroom Assistant to clean. It also helps you keep track of your water usage and uses that data to diagnose any potential problems with your toilet.
To install it:
First, find your water line which usually locates at the bottom left part of the toilet.
Then open the water sensor and clip it onto the water line. Check out the following video: